It is well known in the art that carbon dioxide can be used for extraction of various petroleum fractions. Liquid carbon dioxide is advantageous because of its low cost, non-corrosiveness, non-toxicity and ease of recovery from the extract of raffinate phases. U.S. Pat. No. 2,029,120 discloses the separation of unsaturated hydrocarbons from gas mixtures using liquid carbon dioxide.
U.S. Pat. No. 2,631,966 shows the separation of various lubricating oils with carbon dioxide. As noted therein, liquid CO.sub.2 has certain unusual miscibility relations with hydrocarbons. For example, it dissolves aliphatic and monocyclic aromatic hydrocarbons in preference to polycyclics of the same boiling range.
Typically, CO.sub.2 has not been used alone, but as an aid to other solvents for extraction purposes. These solvents include liquid sulfur dioxide (U.S. Pat. No. 2,034,495) acetone (U.S. Pat. No. 2,246,227) and furfural or phenol (U.S. Pat. No. 2,281,865). A broad description of various solvents that can be used with carbon dioxide can be found in U.S. Pat. No. 2,631,966.
Liquid CO.sub.2 has also been utilized to remove the solvents themselves from the extract phase, as can be found in U.S. Pat. No. 2,646,387.
The ability of CO.sub.2 to function in the above systems will depend upon the solubility of CO.sub.2 with other substances. "Ternary Systems of Liquid Carbon Dioxide" by Alfred W. Frances, Journal of Physical Chemistry 58, 1099 (1954) discloses the solubilities of 261 substances with carbon dioxide.
None of the processes above, however, disclose the ability or process for the dehydration of an aqueous solution by using CO.sub.2.
There are a large number of processes for the production of certain organic chemicals that produce water as a by-product or use steam as a diluent during the reaction. Among these processes is the production of acrylic acid. The aqueous solution resulting from the reaction normally contains from 30-60% acrylic acid with the remainder being mostly water. Various methods have been proposed for removing this water, such as the addition of certain drying agents like calcium chloride as found in U.S. Pat. No. 2,922,815.
Other processes, such as for the production of acrylonitrile, use water as an aid in distilling acrylonitrile from the reactor effluent. This produces an aqueous solution of acrylonitrile that must be dehydrated.
The present invention provides a process for separating water from such aqueous solutions of organic chemicals that can achieve purities upwards of 99%.